The A means it has a special oil pan. Basically, means, the equipment manufacturer specified a different oil pan to meet there needs, which differs from the standard one Kohler provided. All of the oil Pans mount up to the engine block the same, so you can just change it from your engine. I rmemeber an old FOrd Garden tractor, the entire front axle was part of the oil pan for the engine.
a Q means it has balance gears, and is a "quiet" model, S means is electric start,with a geared/bendix type starter. those with generator starters didn"t have the S
I have a K321 AQS in a bolens outside, so is why I remember those 3 letters. theres a couple more letters than can be found I think, the info can be found in the Kohler Service manual available on Kohlers Web site as a free download.